1. Field of the Invention
The invention relates to a non-invasive oximeter arrangement with a clamp-like sensor for a finger, a self-calibrating control unit for generating and processing two electromagnetic waves of specific wavelength and an interposed transfer stretch.
2. Description of the Prior Art
Arrangements of this type are generally used in order to monitor the oxygen saturation in human blood, for example as a check on the vital functions of anaesthetized patients. With the aid of optical scatter measurements on body parts through which there is flow close to the surface, such as for example earlobes or fingertips, a measure of the absorption coefficient of the blood is obtained. The magnitude of the absorption coefficient of blood is greatly dependent on the oxygen content in the case of red light and is almost independent thereof in the case of light in the near infrared range. By measuring the intensity ratio of the light in the two wavelength ranges, a measure of the oxygen saturation of the blood can be obtained.
However, when using this measurement principle, considerable sources of interference and sources of error have been found. Thus, the optically measurable signal is for example greatly dependent on the coupling factors of the optical transmitter and of the receiver to the skin surface, so that the application of the sensor, for example on a finger, is extremely sensitive to movement. Furthermore, the optically measurable signal is dependent on the magnitude of the optically recorded blood volume in the body tissue in question. In addition, for an exact quantitative recording of the oxygen content, knowledge of the ratio of the tissue absorptions, that is to say without blood, is necessary for both wavelength ranges.
An oximeter arrangement is known from the instructions for use of the "Nellcor Pulsoximeter, Modell N-100E, pp. 27, 28, 39 und 40", which is a German translation by Drager Werke AG, Lubeck, September 1986, of the "USERS MANUAL" A 2044, REV A, of the company Nellcor Incorporated, Hayward, Calif., and this oximeter arrangement consists of a clamp-like sensor, an electrical signal transfer section and a signal-processing unit with a microprocessor and displays. The clamp-like sensor has hollow regions which partially receive a finger, the hollow regions are formed by elastic synthetic material and in each case have a window of transparent material, behind which there is arranged in the upper clamp part a double LED and in the lower clamp part a photodiode of large surface area. The LEDs transmit red light in the region of 660 nm and light in the infrared region of 920 nm into the perfused tissue. The portion of the light which is absorbed by the tissue and the non-pulsating blood is different for both wavelengths. However, this portion does not significantly alter during a pulsation. The transmitted residual light is picked up by the photodiode. When the pulsating portion has momentarily disappeared, this intensity represents the initial intensity or reference intensity for the pulsating portion of the absorption. When the pulsating component is present, then the light received by the photodiode at both wavelengths is further reduced by the amount which is absorbed by the pulsating blood at the specific wavelength. With this intensity at both wavelengths, together with the initial intensities at both wavelengths, the ratio of oxygenated blood to the total blood is determined in the control unit. This known oximeter arrangement is expensive and is not suitable for completely solving the abovementioned problems, such as, for example, operating in a strongly electromagnetically disturbed environment in a manner free from interference and safe for the patient. In addition, there is movement sensitivity with respect to the coupling factor, and the effects of extraneous light can falsify the measured result, and there is also the mutually moveable arrangement of the optical transmitter and the optical receiver.
Particularly in view of the important clinical use of treatments involving strong electromagnetic sources, such as, for example, magnetic resonance imaging, in which the use of an oximeter arrangement for monitoring the vital functions of anaesthetized patients in th magnetic resonance imager plays an especially important role, it is extremely important to use an arrangement which, on the one hand, does not disturb the homogeneous electromagnetic field of the imager and, on the other hand, rules out any risks to the patient, such as burn injuries or electric shocks, which can occur as a result of induction, for example in electrical supply lines. This known oximeter arrangement does not satisfy these requirements.